Method of making heater assemblies by wet-settling techniques



METHOD OF MAKING HEATER ASSEMBLIES BY WET-SETTLING TECHNIQUES Filed Oct. 23, 1964 FIG 3 Kenneth Ro/rrer and Rudolf G. Sue/runneh ATTORNEY June 6, 19 67 K. @ROHRER ETAL 3,323,916

Unite States Patent 3,323,916 METHOD OF MAKING HEATER ASSEMBLIES BY WET-SETTLIING TECHNIQUES Kenneth L. Rohrer, Horseheads, N.Y., and Rudolf G. Suchannek, Pasadena, Calif., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Oct. 23, 1964, Ser. No. 406,103 11 Claims. (Cl. 75-207) This invention relates to methods of manufacturing elements for electron discharge devices, and more particularly to improvements in methods for manufacturing electron emitting cathodes for such devices.

In a conventional indirectly heated type cathode, the usual construction of the cathode heater assembly is a sleeve having an external oxide coating or an impregnated matrix on its outside periphery or enclosed end and a filamentary heater coated with a refractory insulation positioned within the cathode sleeve. The electron emissive coating on the enclosed end or periphery of the cathode sleeve is activated by heat radiated from the filament heater which has been suspended within the sleeve. For example, to radiate the necessary heat to the emissive coating during activation of an impregnated tungsten cathode, the filament heater temperature must be maintained at about 500 C. higher than that of the emissive coating. Therefore, due to the amount of heat lost in radiation, the filament heater is operated in a temperature range of approximately 1500 to 1700 C. Unfortunately at these temperatures the refractory insulating coating about the heater filament sometimes becomes unstable; the result is that the refractory coating will become conductive and the successive turns of the filament heater will be shorted.

This problem could be substantially eliminated if the temperature of the filament heater could be lowered sufiiciently. It has been suggested that heat losses could be reduced by minimizing the heat fiow through the cathode sleeve and filament heater supports; however, these proposals are limited by the requirements of accurate cathode alignment and rigid construction. Another solution would suggest that the heat be transferred from the filament heater to the electron emissive layer by conduction rather than radiation with the result that the same amount of heat could be transferred from the filament heater to the electron emissive layer with a much smaller temperature difference between these two elements. To achieve this result, the space between the filament heater and a cathode sleeve must be filled or potted with a material which will provide a good thermal conduction between the cathode sleeve and the filament heater and yet electrically insulate these elements from each other. For optimum result, the surrounding or potting medium should obviously have a good thermal conductivity and be of a continuous nature to provide an efiicient heat path between the filament heater and the electron emissive material. Though the potting medium should be sufiiciently dense to insure efiicient thermal conduction, the potting medium should also be sufliciently porous and have a low vapor pressure to thereby eliminate gas problems after the device has been processed. Further, the potting medium should have a high melting point to withstand the temperatures necessary to energize the electron emissive material.

In prior efforts, filament heaters have been incorporated within a cathode heater assembly by compacting within a die under pressure and at elevated temperatures a thermally conductive potting medium about the filament heater. Even at relatively low pressures and temperatures, the insulating coating about the filament heater may be 3,323,916 Patented June 6, 1967 damaged. Further, difliculty arises due to the sticking of the potting medium to the die rather than to the heater element as the assembly is being withdrawn from the die.

It is, accordingly, an object of this invention to provide an improved method of manufacturing elements for electron discharge devices.

It is a further object of this invention to provide an improved method of manufacturing heater cathode assemblies.

It is another object of this invention to provide an improved method of incorporating a filament heater into a continuous, high thermally conductive medium.

Another object of this invention is to provide a method of embedding a filament heater into a potting medium of sufiiciently porous structure to insure ease in out-gassing the potting medium.

It is a still further object of this invention to provide an improved method of incorporating a filament heater into a potting medium without the use of pressure and at temperatures substantially below the melting temperature of the potting medium.

Briefly, the present invention accomplishes the above cited objects by providing an improved method of embedding a filament heater in a thermally conductive medium within a cathode sleeve or impregnated matrix. More specifically, the filament heater is embedded in a loose mixture of powdered thermally conductive medium to which is added a drying solvent. The powdered medium is then allowed to dry before the assembly is sintered in a reducing atmosphere to thereby form a continuous, relatively dense, thermally conductive medium between the filament heater and the cathode sleeve or impregnated matrix.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention reference may be had to the accompanying drawings, in which:

FIGURE 1 shows an enlarged sectioned view of the heater cathode assembly manufactured in accordance with this invention;

FIG. 2 shows a cross-sectional view of the wire of the filament heater shown in the assembly of FIG. 2; and

FIG. 3 shows a plan view of the filament heater shown in FIG. 1.

Referring now to FIG. 1, there is shown a cathode assembly 40 made in accordance with the method of this invention. An illustrative embodiment of the cathode assembly 40 is comprised of a hollow cathode sleeve 42 made of a material well known in the art such as nickel and having a concave enclosed end 44 with a high density electron emissive coating 46 of a suitable material such as a barium, calcium and strontium oxide thereon. Alternatively, instead of providing the concave end 44 with the emissive coating 46 thereon, a matrix type of cathode element could be provided with a suitable cathode sleeve. Within the opposite end of a cathode sleeve 42 there is a cathode recess 41 wherein there is placed a filament heater 50. As emphasized above, it is desirable to incorporate the filament heater 50 within the cathode assembly 40 by a continuous, thermally conductive potting medium 48; as a result, the filament heater 50 is able to efiiciently transfer thermal energy to the emissive coating 46 (or cathode matrix) to thereby produce an electron stream of higher density. Further, the filament heater 50 may be operated at a lower temperature and thereby prolong the life of a filament heater by preserving the various insulating coatings applied thereto.

The selection of a particular material to be used as a potting medium 48 is a significant aspect of this invention. First, as outlined above, the medium must be a continuous, high thermally conductive material. Further, the medium should he sufficiently porous to allow a thorough outgassing. During the processing of the cathode assembly 40 (as will be explained later), it is necessary to sinter the potting medium 48 within the cathode sleeve 42; therefore, the potting medium 48 should be capable of being sintered at a temperature which will not damage the remaining portions of the cathode assembly 40. Specifically, the sintering of the potting medium 48 should take place at such a temperature which will not deteriorate the cathode sleeve 42, the emissive coating 46 or the insulating coating on the filament heater 50. The following materials have been found to meet these diverse requirements for oxide or matrix type cathodes: nickel, an alloy of nickel and molybdenum, an alloy of nickel and tungsten, molybdenum or tungsten.

After the potting medium 48 has been deposited within the cathode recess 41 by a process to be described, a circular heat shield 52 is placed on top of the potting medium 48 and is secured in that position by retaining tabs 54 which are secured to the inner walls of the cathode sleeve 42 by suitable means well known in the art. Further, terminal portions 59 of the filament heater 50 are withdrawn through the potting medium 48 and the heat shield 52. These terminal portions 59 consist of a heater leg 56 made of a material such as a platinum clad molybdenum about which is wound and spot welded a portion of a filament wire 60 comprising the filament heater 50. About the filament wire 60 there is positioned a heater leg insulator 58; the terminal portions 59 of the filament heater 50 are secured within the cathode recess 41 by tabs 57. Further, connecting wires 37 and 39 are respectively connected to the terminal portions of the filament heater 50 and to the cathode assembly 40.

As shown in FIG. 3, the filament heater 50 consists of the filament wire 60 which has been wound in a noninductive reverse helix. As shown in FIGS. 1 and 3, the terminal portions 59 of the filament heater 50 are taken off at positions 90 apart from each other.

Referring now to FIG. 2, the filament wire 60 may be composed of a central core 61 of tungsten with successive concentric layers 62 and 64 of molybdenum and aluminum oxide respectively. As disclosed in the Patent No. 3,005,926 to Horner et al., when tungsten or rhenium is placed in immediate association with a nickel material, the tungsten or rhenium will become embrittled when subjected to high temperatures. Therefore, in the specific embodiment incorporating a nickel potting medium 48, the tungsten core 61 has been coated with the continuous layer of molybdenum of approximately .4 microns in thickness. The layer 62 may be applied by a suitable method known in the art such as by reducing an electrolytically deposited layer of molybdenum oxide on the tungsten wire at 1200 C. for one minute. The concentric layer 64 of aluminum oxide is then applied about the wire 60 in order to provide an insulating layer. Though at present it is believed that molybdenum provides the greatest protection for tungsten against nickel embrittlement. it has been found that rhodium, platinum, rhenium, irridium and osmium provide a significant protection against embrittlement due to the presence of nickel.

Many attempts were made to find an appropriate process or method which would provide a dense potting medium 48 and at the same time eliminate any adverse efects of such a process on the heater cathode assembly 40. High sintering temperatures and long sintering times must be avoided because of heater embrittlement, deformation of the cathode sleeve, leaching out of additives from impregnated electron emissive layers, and the formation of eutectic mixtures which may strip or remove the insulation coating from the heater filament thereby causing shorts. Further, loosely packed powders will shrink excessively and demand too high a temperature and sintering time. It is particularly noted that either cold or hot pressing are not mechanically feasible because even at low pressures or low temperatures, cracking of the insulating coatings 64 and 62 and sticking of the filament heater 50 and the potting medium 48 to the die will occur. Likewise, slip casting and slurry techniques have been found undesirable, and further these techniques yield a soft, very porous medium after sintering. With these limitations in view, a wet-settling method was invented to yield a very dense and adherent medium which is easily outgassed and which provides a high thermally conductive medium.

In an illustrative embodiment of the method, a loose metal powder of approximately 200 mesh is heaped and leveled on the bottom of the cathode recess 41. A coated heater filament 50 with leg insulators 58 in place as described above is then positioned on top of the powder. An additional amount of powder is added to the desired height. Next, a fast drying organic solvent, such as isopropanol, mixed with an approximately equal volume of deionized water is poured into the cathode recess 41 and over the powder containing the filament heater 50 until the liquid level is above the level of the powder. It is noted that for less critical requirements, that water alone could be used as the drying solvent. Next, the metal powder is allowed to stand at room temperature until the lquid level has sunk below the level of the powder. Then the cathode sleeve 42 is dried in an oven at approximately C. for approximately 5 minutes. It is noted that the use of water alone may require longer drying time. Finally, the cathode sleeve 42 is sintered in a suitable reducing atmosphere such as cracked ammonia or hydrogen. In specific embodiments, potting mediums of mixtures of 52% nickel and 48% molybdenum, and 52% nickel and 48% tungsten have been sintered for approximately 5 minutes at temperatures of approximately 1200 and 1300 C. respectively. For a potting medium of nickel, the cathode sleeve was sintered at approximately 1100 C. for approximately 5 minutes. Further, pure molybdenum or tungsten have been sintered for approximately 5 minutes at 1350 C.

Though the exact physical mechanism of this process is not known, the resultant product is a very dense, continuous potting medium which adheres well to the cathode recess 41. At present, it is believed that this process produces a very close interparticle adherence or bonding; there is evidence that oxides may be formed from the water present to promote additional fusion and cementation of the particles upon reduction.

It will therefore be apparent that there has been disclosed a method of manufacturing a cathode assembly capable of efliciently transmitting thermal energy from the filament heater to the emissive coating or matrix cathode to thereby effect a high density stream of electrons. The method herein disclosed is capable of forming the powdered metallic medium into an extremely dense homogeneous material without the use of pressure or resulting danger of cracking the insulation surrounding the filament heater.

While there has been shown and described what are presently considered to be the preferred embodiments of this invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore that the invention be limited to the specific arrangements shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

We claim as our invention:

1. A method of producing an embedded-heater cathode assembly for an electron discharge device comprising the steps of inserting a heater element into a cathode receptacle and depositing within said cathode receptacle a measured amount of thermally conductive particles to envelop said heater element therein, said particles being suitable for use in an evacuated electron discharge device, adding a volatile liquid capable of being substantially completely fugitive from said particles, allowing said cathode receptacle to stand for a s-ufficient time to allow said volatile liquid to seep into and to compact and bond said particles, and sintering said particles for a time sufficient to substantially completely sinter said particles into a dense, continuous medium.

2. A method of producing an embedded-heater cathode assembly for an electron discharge device comprising the steps of inserting a heater element having an electrically insulating coating thereon into -a cathode receptacle and depositing within said cathode receptacle a measured amount of metallic particles to envelop said heater element therein, said metallic particles being thermally conductive and capable of being sintered at a temperature below that which will adversely affect said insulating layer, adding a volatile liquid capable of being completely fugitive from said metallic particles and allowing said cathode receptacle to stand for a suflicient time to allow said volatile liquid to seep into and to compact and bond said metallic particles, and sintering said metallic particles for a period of time sufficient to substantially completely sinter said metallic particles into a dense continu ous medium without adversely affecting said insulating layer.

3. A method of producing an embedded-heater cathode assembly for an electron discharge device comprising the steps of inserting a heater element into a cathode receptacle and depositing within said cathode receptacle a measured amount of metallic particles to envelop said heater element therein, said metallic particles being thermally conductive and suitable for use in an evacuated electron discharge device, adding a volatile liquid capable of being completely fugitive from said metallic particles and allowing said cathode receptacle to stand for a sufficient time to allow said volatile liquid to seep into and to compact and bond said metallic particles, and sintering said metallic particles at a temperature below the melting temperature of said metallic particles to thereby insure that said insulating coating is not adversely afiected.

4. A method of producing an embedded-heater cathode assembly for an electron discharge device comprising the steps of inserting a heater element into a cathode receptacle and depositing within said cathode receptacle a measured amount of metallic particles to envelop said heater element therein, said metallic particles being thermally conductive and suitable for use in an evacuated electron discharge device, adding a volatile liquid capable of being completely fugitive from said metallic particles, allowing said cathode receptacle to stand for a sufficient time to allow said volatile liquid to seep into and to compact and bond said metallic particles, heating said metallic particles to a sufiicient temperature for a suffi-cient length of time to drive off substantially all of said volatile liquid, and sintering said metallic particles for a time sufficient to substantially completely sinter said metallic particles into a dense, continuous medium.

5. The method of claim 4 wherein said volatile liquid comprises a solution of isopropanol and water, and said cathode receptacle is dried at a temperature of approximately C. for a period of approximately 5 minutes.

6. The method of claim 4 wherein said cathode receptacle is sintered for approximately 5 minutes.

7. The method of claim 6 wherein said metallic particles comprise a mixture of approximately 52% nickel and 48% molybdenum, and the cathode receptacle is sintered at a temperature of approximately 1200 C.

8. The method of claim 6 wherein said metallic particles comprise approximately 52% nickel and 48% tungsten, and the cathode receptacle is sintered at a temperature of approximately 1300 C.

9. The method of claim 6 wherein said metallic particles comprise molybdenum, and the cathode receptacle is sintered at a temperature of approximately 0 C.

10. The method of claim 6 wherein said metallic particles comprise tungsten, and the cathode receptacle is sintered at a temperature of approximately 1350 C.

11. The method of claim 6 wherein said metallic particles comprise nickel, and the cathode receptacle is sintered at a temperature of approximately 1100" C.

References Cited UNITED STATES PATENTS 2,776,887 1/1957 Kelly 75-222 X 2,798,182 7/1957 Costa 313--340 2,830,218 4/1958 Beggs.

2,975,322 3/1961 Cockrill 313346 3,117,249 1/1964 Winters.

3,227,911 1/1966 Heil 313--337 CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

A. J. STEINER, Assistant Examiner. 

4. A METHOD OF PRODUCING AN EMBEDDED-HEATER CATHODE ASSEMBLY FOR AN ELECTRON DISCHARGE DEVICE COMPRISING THE STEPS OF INSERTING A HEATER ELEMENT INTO A CATHODE RECEPTACLE AND DEPOSITING WITHIN SAID CATHODE RECEPTACLE A MEASURED AMOUNT OF METALLIC PARTICLES TO ENVELOP SAID HEATER ELEMENT THEREIN, SAID METALLIC PARTICLES BEING THERMALLY CONDUCTIVE AND SUITABLE FOR USE IN AN EVACUATED ELECTRON DISCHARGE DEVICE, ADDING A VOLATILE LIQUID CAPABLE OF BEING COMPLETELY FUGITIVE FROM SAID METALLIC PARTICLES, ALLOWING SAID CATHODE RECEPTACLE TO STAND FOR A SUFFICIENT TIME TO ALLOW SAID VOLATILE LIQUID TO SEEP INTO AND TO COMPACT AND BOND SAID METALLIC PARTICLES, HEATING SAID METALLIC PARTICLES TO A SUFFICIENT TEMPERATURE FOR A SUFFICIENT LENGTH OF TIME TO DRIVE OFF SUBSTANTIALLY ALL OF SAID VOLATILE LIQUID, AND SINTERING SAID METALLIC PARTICLES FOR A TIME SUFFICIENT TO SUBSTANTIALLY COMPLETELY SINTER SAID METALLIC PARTICLES INTO A DENSE, CONTINUOUS MEDIUM. 